The net rate of carbon storage in forests varies as a result of interannual
variability in rainfall, temperature, and disturbance regimes. For example,
transient simulations with the Terrestrial Ecosystem Model (TEM) suggest that
forests of the Amazon basin during hot, dry El Niño years were a net
source of carbon (of as much as 0.2 Gt C yr-1), whereas in other
years they were a net sink (as much as 0.7 Gt C yr-1) (Tian et al.,
1998). Notably, source and sink strength varied across the basin, indicating
regional variation. Similarly for the United States, the net rate of carbon
storage varied from a net source of about 0.1 Gt C yr-1 to a net
sink of 0.2 Gt C yr-1 (Schimel et al., 2000). The rate of carbon
accumulation in undisturbed forests of the Amazon basin reported by Tian et
al. (1998) was approximately equal to the annual source from deforestation in
the same area. For the United States, Houghton et al. (1999)using inventory
dataestimate a sink of 0.35 Gt yr-1 during the 1980s as a result
of forest management and regrowth on abandoned agricultural lands, whereas Schimel
et al. (2000) indicate that 0.08 Gt yr-1 stored from 1980 to 1993
could have been a result of carbon fertilization and climate effects. Thus,
for the United States, the rate of carbon accumulation in forest regrowth on
abandoned agricultural and harvested managed forest lands appears to be as large
as or larger than the direct effects of CO2 and climate (Schimel
et al., 2000). The SAR (Kirschbaum et al., 1996) suggests that modeled trends
described above are unlikely to continue under projected climate change and
high elevated CO2 concentrations. However, climatic variability (including
ENSO events) is likely to increase under a changed climate (see Table
3-10), which may increase interannual variation in regional carbon uptakeas
demonstrated by Tian et al. (1998) and Schimel et al. (2000).

5.6.3.2. Responses and Impacts: Timber and Non-Wood Goods
and Services

5.6.3.2.1. Response to locally extreme events

In addition to fire and insect predation, other episodic losses may become
increasingly important in response to locally extreme weather events. For example,
in Europe, wind-throw damage has appeared to increase steadily from negligible
values prior to about 1950; wind-throws exceeding 20 million m3 have
occurred 10 times since then (UN-ECE/FAO, 2000b). Losses in 1990 and 1999 were
estimated at 120 million and 193 million m3, respectivelythe
latter equivalent to 2 years of harvest and the result of just three storms
over a period of 3 days (UN-ECE/FAO, 2000b). The unusual 1998 icestorms in eastern
North America caused heavy damage to infrastructure as well as large areas of
forest, the extent of which is still being assessed (Irland, 1998).

Financial returns to forest landowners decline if these episodic events (including
fire and insect predation) increase (Haight et al., 1995), although the impact
on global timber supply of such episodic losses is unlikely to be significant.
Local effects on timber and non-wood goods and services may be significant,
although timber loss may be ameliorated through salvage logging. Moreover, the
location of the loss is particularly important for non-wood products and services.
For example, forest fires in recreational areas are known to have an impact
(Englin et al., 1996).